1. Field of the Invention
The present invention is concerned with ionizing radiation dosimeters preferably including extremely thin, solid, self-sustaining thermoluminescent material bodies as a part thereof, along with a corresponding method of manufacturing the dosimeters. More particularly, it is concerned with such a dosimeter having a TLD chip or the like which is extremely thin (less than about 0.25 millimeters) and which is produced by a novel technique of sanding initially thick TLD bodies, the sanding procedure also serving to drastically reduce initial nonradiation-induced thermoluminescence exhibited by the body. Indeed, the present invention discloses the first solid, substantially pure TLD material having the requisite thinness for highly accurate beta dosimetry.
2. Description of the Prior Art
Ionizing radiation is routinely measured by exposing a thermoluminescent dosimeter to the radiation and thereafter measuring the light output of the dosimeter when it is heated. Various thermoluminescent (TL) materials are known, the preferred ones being crystalline ionic salts of the alkali metal and alkaline earth metals, with the most common example being lithium fluoride. Radiation dosimetry using crystalline dosimeters is the most widely utilized method for monitoring the radiation to which persons, in an ambience of potentially injurious radiation, have been exposed over a known time interval, ranging from a few minutes to several weeks.
Typically, persons likely to be exposed to radiation carry a dosimeter fixedly mounted in a support such as a card which is subsequently read by being positioned for heat conductive contact with a heated element in a light-tight chamber. The heated dosimeter luminesces and, with the aid of a photo-multiplier tube, generates a reading indicative of the absorbed dose to which the dosimeter and the person carrying it have been subjected. It will be apparent that where a large number of persons are susceptible to radiation in their working environment, the absorbed dose to which each has been subjected must be monitored relatively frequently, and hence the use of radiation dosimeters in this context is of extreme importance.
Personnel dosimeters have been suggested in the past in order to meet the need for convenience, accuracy and ease of reading. Generally speaking, such personnel dosimeters include a wafer, ribbon, rod or chip of thermoluminescent material mounted with an appropriate support and encased within a convenient clip-on card or badge. Advantageously, the support for the thermoluminescent body is approximately tissue equivalent with respect to ambient radiation, so that personnel dosimetry results are as accurate as possible.
Another class of thermoluminescent dosimeters utilizes a high temperature resistant support material, such as a metal, to measure radiation in high intensity fields, often also at elevated temperatures.
Various types of TLD bodies have also been proposed for use in composite dosimeters of the type described above. For example, single crystals of thermoluminescent material have been used, but these display a very non-uniform distribution of thermoluminescent sensitivities. Accordingly, it has been found that by grinding up many such crystals and thoroughly mixing the resultant powder, a reasonably uniform thermoluminescent sensitivity can be achieved. Because powder acts essentially like a fluid, it adopts the shape of the holder in which it is placed, thus providing a degree of flexibility in the choice of dosimeter size and shape.
Another form of conventional thermoluminescent body comprises a ribbon or rod of thermoluminescent material formed by extrusion and/or hot pressing of an initial powder. In the case of extruded bodies, the fused polycrystalline material is extruded through an appropriately shaped die, cut and polished. A variety of sizes and shapes are available in connection with extruded and hot pressed thermoluminescent bodies, but the currently most popular ones are 3.times.3.times.0.9 millimeter chips and 1.times.1.times.6 millimeter micro-rods.
So-called PTFE-based dosimeters are available, which are produced by compressing and heating a homogeneous mixture of fine-grain phosphor powder (such as LiF, typically 10 micrometers in diameter) and PTFE powder to a temperature above the softening temperature of PTFE (327.degree. C.) in a mold. By this means, it is possible to form an intimate matrix of phosphor and PTFE. By varying the loading fraction of the phosphor powder the sensitivity of the dosimeter can be changed. PTFE-based dosimeters are available with different phosphors and in a variety of geometries. Discs are available with diameters of 2-13 millimeters and thicknesses of 0.02-0.50 millimeters. Micro-rods have also been produced in various lengths and one millimeter diameter. PTFE-based dosimeters cannot be annealed in bulk at temperatures greater than 300.degree. C., and moreover, because of the PTFE filler, have higher minimum detectable radiation doses as compared with solid thermoluminescent bodies of the same thickness. Accordingly, PTFE-based dosimeters have not achieved the degree of popularity of solid body thermoluminescent dosimeters.
A type of composite thin-element TLD is described in "Construction Of A Composite Thin-Element TLD Using An Optical-Heating Method", Health Physics, Vol. 43, No. 3, pp. 383-390, September 1982, by O. Yamamoto et al. This paper describes a dosimeter made up of a mono-layer of phosphor granules of about 0.09 millimeters diameter formed on a substrate polyimide film of 11 mg/cm.sup.2 thick polyimide monomers as a binder. A thin carbon layer is coated onto the opposite side of the polyimide film to increase the absorbency for irradiation. A transparent Teflon FEP film 22 mg/cm.sup.2 thick covers the phosphor layer with a gap of 0.5 millimeters to keep out dust, moisture, sweat and the like. As can be appreciated from the foregoing, the dosimeter described by Yamamoto et al. is in the form of a plurality of extremely minute powder-like particles of either lithium borate or calcium sulfate supported on an adhesive light-tight substrate.
Other references describing various types of dosimeters include: U.S. Pat. Nos. 3,320,180, Re. 28,340, 3,809,901, 3,894,238 and 4,039,834, and Thermoluminescence Dosimetry by A. F. McKinlay, Adam Hilger Ltd., pp. 51-58, 1981.
Most present day commercially available personnel dosimeters, particularly those employing solid body thermoluminescent material, are deficient in that they underestimate, to a greater or lesser degree, absorbed dosages of low energy beta radiation when calibrated according to American National Standard N13.11 (1982). The reason for this deficiency is that the thermoluminescent materials employed are too thick, typically by an order of magnitude or more. In order to be accurate, the active thermoluminescent body must be thin enough so that its thickness is less than the range of nearly all of the beta emitters of interest. If this condition is not met, then the active dosimeter volume changes with variations in the energies of the beta particles striking the dosimeter. Since many conventional beta dosimeters are calibrated with high energy particles or gamma rays, the calibration inherently assumes the entire dosimeter volume is actively absorbing the radiation energy. Again, this will not be a valid assumption if the dosimeter thickness exceeds the beta range, and it may result in a very severe under estimation of the beta dose. Thus, a thin layer dosimeter is required in order to achieve the desired energy response.
The problem of providing a thin thermoluminescent body for use in a personnel or other dosimeter has presented a significant problem to prior researchers. For example, as noted in Thermoluminescence Dosimetry, supra, p. 91, "thin, robust skin dosemeters are particularly difficult to produce." In addition, the following references deal with beta dosimetry and the problem of providing thin thermoluminescent materials so as to achieve an appropriate low beta energy response: A. Koczynski and M. Wolska-Witer, "Graphite-Mixed Non-Transparent LiF and Li.sub.2 B.sub.4 O.sub.7 :Mn TL Dosimeters Combined With A Two Side Reading System For Beta-Gamma Dosimetry" Central Laboratory for Radiological Protection, Warsaw, Poland; T. F. Gesell, "A Personnel Beta Dosimetry Method for Reducing Energy Dependence", IDO-12090 (1979); M. Marshall and J. Dooherty, "Measurement of Skin Dose from Low Energy Beta and Gamma Radiation Using Thermoluminescent Discs", Phys. Med. Biol., Vol. 16, No. 3, pp. 503-510, 1971; and D. Lowe, J. R. A. Lakey and B. J. Tymons, "A New Development in Skin Dosimetry", Nuclear Instruments and Methods 169, pp. 609-612, 1980.
The recognized need for accurate beta dosimetry has led to the promulgation of industry standards which are virtually impossible to meet using conventional thick dosimeters. That is to say, standards have been set which in effect require extremely thin TLD's, but prior to the present invention commercial production of substantially pure, solid, self-sustaining thin TLD's has not been achieved.
In view of the foregoing, there is a decided and heretofore unresolved need in the art for a radiation dosimeter including an extremely thin, solid, essentially pure crystalline thermoluminescent body giving more accurate low energy response to all forms of ionizing radiation.